Method and apparatus for implementing a lock-free skip list that supports concurrent accesses

Abstract

One embodiment of the present invention provides a system that supports concurrent accesses to a skip list that is lock-free, which means that the skip list can be simultaneously accessed by multiple processes without requiring the processes to perform locking operations. During a node deletion operation, the system receives reference to a target node to be deleted from the skip list. The system marks a next pointer in the target node to indicate that the target node is deleted, wherein next pointer contains the address of an immediately following node in the skip list. This marking operation does not destroy the address of the immediately following node, and furthermore, the marking operation is performed atomically and thereby without interference from other processes. The system then atomically modifies the next pointer of an immediately preceding node in the skip list to point to an immediately following node in the skip list, instead of pointing to the target node, thereby splicing the target node out of the skip list.

Description

RELATED APPLICATION[0001]This application hereby claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application No. 60 / 456,792, filed on 21 Mar. 2003, entitled “Practical Lock-Free Skip List,” by inventor Paul A. Martin.BACKGROUND[0002]1. Field of the Invention[0003]The present invention relates to the design of lookup structures within computer systems. More specifically, the present invention relates to a method and apparatus for implementing a lock-free skip list that supports concurrent accesses within a computer system.[0004]2. Related Art[0005]A skip list is a dynamically sized sorted linked list that offers logarithmic time performance for searching, inserting, and deleting elements. William Pugh developed a basic design for a skip list to be used by a single thread (see “A Skip List Cookbook,” by William Pugh, University of Maryland Institute for Advanced Computer Studies UMIACS Technical Report No. UMIACS-TR-89-72.1).[0006]A skip list is neither a single list (...

AI Technical Summary

Benefits of technology

[0012]One embodiment of the present invention provides a system that supports concurrent accesses to a skip list that is lock-free. The term “lock-free” means that the skip list can be simultaneously accessed by multiple processes without requiring the processes to perform locking operations (non-blocking), and furthermore that a finite number of operations by a thread will guarantee progress by at least one thread (lock-free). During a node deletion operation, the system receives reference to a target node to be deleted from the skip list. The system marks a next pointer in the target node to indicate that the target node is deleted, wherein next pointer contains the address of an immediately following node in the skip list. This marking operation does not destroy the address of the immediately following node, and furthermore, the marking operation is performed atomically and thereby without interference from other processes. The system then atomically modifies the next pointer of an immediately preceding node in the skip list to point to an immediately following node in the skip list, instead of pointing to the target node, thereby splicing the target node out of the skip list.

Problems solved by technology

However, when large numbers of processes access a skip list concurrently, contention for locks can become a serious impediment to system performance.

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